21721224 has 32 divisors (see below), whose sum is σ = 62061120. Its totient is φ = 6206016.

The previous prime is 21721211. The next prime is 21721229. The reversal of 21721224 is 42212712.

It is a Harshad number since it is a multiple of its sum of digits (21).

It is a nude number because it is divisible by every one of its digits.

It is a self number, because there is not a number n which added to its sum of digits gives 21721224.

It is not an unprimeable number, because it can be changed into a prime (21721229) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 64479 + ... + 64814.

It is an arithmetic number, because the mean of its divisors is an integer number (1939410).

Almost surely, 2^21721224 is an apocalyptic number.

21721224 is a gapful number since it is divisible by the number (24) formed by its first and last digit.

It is an amenable number.

21721224 is an abundant number, since it is smaller than the sum of its proper divisors (40339896).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21721224 is a wasteful number, since it uses less digits than its factorization.

21721224 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 129309 (or 129305 counting only the distinct ones).

The product of its digits is 448, while the sum is 21.

The square root of 21721224 is about 4660.6033944115. The cubic root of 21721224 is about 279.0153509622.

Adding to 21721224 its reverse (42212712), we get a palindrome (63933936).

It can be divided in two parts, 21721 and 224, that added together give a triangular number (21945 = T₂₀₉).

The spelling of 21721224 in words is "twenty-one million, seven hundred twenty-one thousand, two hundred twenty-four".